After distinguishing between the characteristics of seas and oceans, the thermohaline circulation of seas in arid and humid climates is discussed, along with the effects of winds and tides. The isotopic distribution in Mediterranean seas and its relationship with water balance and air/sea interaction processes is described and examples of isotope hydrography from the Gulf of Aquaba, the Red Sea, the Persian Gulf, the Mediterranean, the Black Sea and the Baltic are given. Finally, the isotopic hydrography of highly evaporated marine lagoons is reviewed. -A.W.Hall